A. The Medical Field
Generally speaking, computerized tomography is a modern technique initially developed for use in the medical field to provide a non-invasive means for revealing internal organs and tissues of the human body in cross-section to aid in medical diagnosis, surgery, etc. Essentially, an X-ray beam (also including, in certain instances gamma radiation) is passed through the body and the attenuation difference between the transmitted beam and the detected beam is sensed by a detector system, digitized and stored in a computer. The beam is then rotated in one plane to a different angular position and the attenuated beam's energy at that position similarly recorded. The process continues for 360.degree., at which time the computer images the data recorded to develop a two dimensional picture of a cross-sectional slice taken through the patient which corresponds to the plane in which the X-ray beam was rotated. The X-ray beam is then transversely moved and the process repeated to develop another picture of a cross-sectional slice of the patient. By taking a plurality of such transversely spaced slices and stacking them one on top of the other, a three dimensional transparent view can be constructed by the computer.
The first commercial application of computerized axial tomography (CAT) is attributed to Hounsfield in 1972 and used a pencil beam with a single detector. The beam and detector were simultaneously rotated and then linearly translated to develop an appropriate scan of the organ. This is conventionally referred to as the first generation scanner. To reduce the time required for the scan, the pencil beam ray was replaced by a beam of X-rays orientated in a thin fan-shaped pattern with the attenuated rays in the fan sensed by a plurality of detectors on the opposite side of the body. Various detector arrays and detector-beam movements were subsequently developed in second, third and fourth generation scanners, all of which were directed to increasing the speed of the scan. In all scanners of the first through fourth generation, a three dimensional view of the scanned object was obtained by, first computing an image of a cross-sectional slice and then stacking such slices to construct a three dimensional transparent or translucent image.
There are inherent problems in medical scanners of the first through fourth generation which preclude their use in industrial applications. Conceptually, any system that rotates a beam to obtain several one dimension images which are subsequently combined to produce a cross-sectional "slice" and which then translates the beam to build a plurality of slices requires a scan-compute time which is simply too slow for industrial inspection purposes. Also, any fan beam in reality has a finite width or a depth while the cross-sectional slice is assumed to be a planar line. Accordingly, there are numerous prior art patents relating to detectors, collimators, scatter shields, etc., which have been designed to reduce the beam width and improve image resolution. Finally, in three dimensional imaging, the computer uses various formulae, assumptions and corrections to calculate what the irradiated object looks like in the space between the slices. Where high resolution and accuracy is required, numerous slices must be taken to build an accurate three dimensional image.
In addition to such problems, first through fourth generation scanners cannot accurately image certain moving organs such as the heart. Accordingly, there have been recent developments in the medical field reported in the Robb et al article which utilize a cone beam instead of a fan beam and an area detector in place of the one dimensional detector arrays to provide such a system.
It is known that a cone beam of X-rays can be developed and that such beam can be projected onto a fluoroscopic screen or recorded on photographic film for two dimensional imaging. A number of papers have presented formulae for cone beam back projections which are used to construct the images in a computed tomographic system. Despite the number of papers, the use of cone beams in three dimensional X-ray computed tomographic systems has only been reported as successfully practiced in the medical scanner(s) described in the Robb et al article. In the Mayo Clinic scanner described in Robb's paper, multiple X-ray tubes are placed around a 160.degree. arc of a circular gantry which mechanically rotates about the patient while carrying a diametrically opposed fluorescent screen. The screen records two dimensional shadow data for each of the X-ray cone beam sources which are described as being fourteen in number. The orientation of the object to be scanned is such that the distance from the source of radiation to the object is significantly greater than the distance from the object to the detector so that the transmitted beams in the cone striking the screen can be viewed as parallel beams to permit reconstruction in the manner of a fan beam slice system. Conceptually, the system developed at the Mayo Clinic is sound and represents a significant advance in the medical field permitting heart studies and the like. The geometrics of the system are such that while adjacent cone beams can be formed to uniformly irradiate an object, the attenuated beams in the adjacent or fringe areas will interfere with one another before striking the detector. For this reason, the fluorescent screen is positioned close to the patient. While the interference can be compensated for at the detectors, commercial objects having high mass densities would produce weak fringe signals making it difficult to obtain accurate high resolution signals or increasing the scan time, etc.
In the related nuclear medicine field, Technicare U.S. Pat. No. 4,302,675 discloses an adjustable collimator in combination with a scintillation camera where the pinhole axes in the collimator are movable to record various incident angles of gamma rays emitted from an object to construct a simulated three dimensional image of the object. Also, Hounsfield U.S. Pat. No. 4,322,684 discloses a three dimensional imaging technique utilizing nuclear magnetic resonance where resonance is induced in a plurality of planar slices through an object which is rotated about a first axis and then further rotated about a second axis. The slices are then integrated to obtain a three dimensional view. Neither nuclear medicine application uses X-rays emitted from a point source travelling in straight lines.